This specification relates generally to information retrieval based on search queries. The Internet provides access to a wide variety of resources, for example, video files, image files, audio files, or Web pages including content for particular subjects, book articles, or news articles. A search system can select one or more resources in response to receiving a search query. A search query is data that a user submits to a search engine to satisfy the user's informational needs. The search queries are usually in the form of text, e.g., one or more query terms. The search system selects and scores resources based on their relevance to the search query and on their importance relative to other resources to provide search results that link to the selected resources. The search results are typically ordered according to the scores and presented according to this order.
Many search engines allow a user to perform searches using natural language queries. Natural language queries often include terms that are commonly used in full sentences but are not essential to the concepts for which the user wishes to obtain more information. These common terms can sometimes be identified based on their high frequencies of occurrence in a document corpus, and given less weight during a search when presented with other, less frequent search terms.
Even with the generalized term importance evaluation describe above, however, a search engine may return results that are not of interest to the user, or do not fully satisfy the user's need for information. There can be a number of reasons that the search engine may provide such results. For example, the weights given to the terms in the query may not have reflected the user's specific interests (e.g., when a word in a query that is deemed more important by the user is attributed less weight by the search engine than other words in the query, or vice versa). Sometimes, the query itself may be a poor expression of the information needed, or the query may have included terms that are extraneous, misspelled, or used in an unconventional way.